Solar Cells Power Underwater Sensors

The Navy is exploring the use of solar cells to power sensors underwater as part of research across the US military to develop alternative, environmentally friendly, and efficient sources of energy.

Scientists at the US Naval Research Lab's (NRL) Electronics Science and Technology Division have had success using underwater photovoltaic research to develop solar cells that can operate electronic sensors at depths of up to 9m, or about 30ft. Photovoltaics involves generating electrical power by converting solar radiation into direct current electricity.

The research came about due to the need for long-endurance power sources to fuel underwater autonomous systems and sensors. They currently rely on power sources onshore, batteries, or solar power supplied by a source above the water, but these are limiting and inefficient, according to the NRL.

Graph shows the power density of GaInP and crystalline silicon cells, underwater, as a function of depth. Scientists found that by using GaInP cells, they could generate more power from sunlight filtered through water. (Source: US Naval Research Lab.)

The task of using solar energy underwater was not without its challenges, however. Scientists had explored the use of photovoltaics before, but with solar cells that were optimized for the terrestrial solar spectrum. Therefore, these systems had only limited power because of the lack of sunlight that reached them underwater. To solve that problem, scientists designed the solar cells using different materials to match the wavelength range of the less-intense solar radiation underwater.

Previous solar cells were based on crystalline silicon or amorphous silicon, which are well matched to the wavelength of terrestrial sunlight. To take advantage of solar energy in low-light conditions, however, scientists switched their focus to the design of high-quality gallium indium phosphide (GaInP), a higher bandgap material that performs better in low-light conditions.

The filtered spectrum of sun underwater uses the blue/green portion of the light spectrum more than any other, researchers found. GaInP has high quantum efficiency in wavelengths between 400 nanometers and 700 nanometers and intrinsically low dark current, a spectrum that allows it to perform much more efficiently.

By creating solar cells using GaInP, researchers have been able to generate 7W per square meter with solar cells at a maximum depth of 9.1m, or about 30ft. This performance shows that the cells can be used to power sensor systems at typical depths close to shorelines, according to the NRL.

The Navy is not the only military branch exploring the use of alternative energy to provide more cost-effective and environmentally-friendly power sources. The Army also is leveraging solar power, specifically in portable renewable energy microgrids that use both the sun and wind to generate electricity.

Elizabeth, this can go a long way toward solving a logistic challenge for naval sensors. I assume that some of them might be associated with ASW technology. These have to be around for a long time. An autonomous power source like this will decrease cost to service and enhance security, since they do not have to have their batteries replaced by ships that can be tracked. The decision to use a different material for the solar cells is also interesting. All too often we use materials, like silicon, that are familiar and easy to work with.

Great story. It's been said that in some (northern) geographical areas of the U.S. solar cells produce usable energy about 15-20% of the time. I wonder how much of the time an underwater solar cell can produce usable current.

I'm sure that using the correct materials that an efficient solar cell can be made. The devil is in the packaging. I have never seen a shallow underwater surface that was not covered with silt, algae or some manner of marine growth.

Thanks for reporting this. There are several different materials that can be used for PV solar, as well as for non-PV solar technologies. The predominance of silicon for PV solar, most particularly crystalline silicon, happened mostly because it was a cheaper, ubiquitous material and because the entire sourcing and manufacturing process could be easily integrated into the existing manufacturing infrastructure using existing processes, knowledge and equipment. Solar cells based on gallium arsenide (GaAs), such as triple junction GaAs, and other forms of gallium like GaInP are highly efficient, but the material cost is very expensive.

large ships EASILY have a draft of 29 feet. What about subs? This gives them something to bump into or grind up with the props. Seems to me that the thing will have to be anchored to the bottom anyway (like a mine) so why not just use the ocean movement (spelled waves)?

I'm REALLY skeptical about the amount of energy (Watts) you can get from the things as in the already mentioned seaweed, etc. not to mention barnacles that seem to like ship hulls.

Elizabeth—great story.I have my reservations about the degree of ultimate success for this technology but then again stranger things have happened.I think tomw is on to something when he mentions the debris that can accumulate on a flat plate collector.I will say this; the amount of energy for the depth involved is impressive.I wonder if the degree of salinity has a great effect on the transference of light to the collector. Robatnorcross mentioned subs an ocean-going ships interfering with the installations.How about curious fish (big fish) and migrating schools of "whatever".At any rate, certainly an interesting topic to follow.Thank you for the information.

Interesting thought. I was wonder what type of sensor and systems are we talking about? My first thought was this was going to be part of a large array of sensors which only recorded a few KB of data. If thats' the case, they would be somewhat disposable so if i ship or curious fish took some out there wouldn't be an issue.

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